Empirical Relation between Drying and Wetting Branches of Soil Water Retention Curve by Using van Genuchten Model

Quantification of soil water retention curve (WRC) is essential to study soil water movement in unsaturated soils. Direct measurement of soil moisture and matrix potential is too costly and time consuming. Moreover، due to the phenomenon of hysteresis، moisture in the drying branch cannot be used on wetting branch. Therefore، using the indirect method to find the relationship between the two branches of the WRC is needed. Development and use of the pedotransfer function (PTF) is one of the indirect methods. The purpose of this study was to provide a function for the parameters of van Genuchten model estimation in wetting branch using drying branch data. Required information in this research was collected from soil data base (Unsuda). These data include moisture retention curve data of drying and wetting and soil bulk density of 21 soil samples classified in sandy (10 samples)، clay loam and loam (7samples)، and silty clay loam (4samples) texture. To provide the functions، all samples were used. Results showed that nw in van Genuchten model was estimated with good accuracy، while the introduced function for the parameter αw had lower performance. The model obtained for estimation of the parameters of van Genuchten model in the wetting branch of WRC had a higher accuracy in low potentials. Results show that PTF are more powerful than Parlange hysteresis model for estimation of WRC in the wetting branch.